A satellite image of Cyclone Phailin off the east coast of India on Oct. 11, 2013.

So what are cyclones and how do they develop?

Derived from the Greek word cyclos, meaning coiling of a snake, cyclones are described by weather officials as low pressure rotational systems that whirl violently across the sea surface.

They are triggered as winds caused by the Earth’s rotation combine with low pressure areas over the ocean.

Tropical cyclones are propelled by pressure. Their energy comes from the warm water and humid air over tropical seas. Clouds, which can cover hundreds of miles, accumulate as they pick up moisture from the sea. Low pressure areas pull the clouds toward land.

Along their path, well-developed cyclones release the same amount of energy as 100 hydrogen bombs, weather officials say.

Cyclones are usually classified by their wind speeds. The 1999 “Orissa Super Cyclone,” which caused as many as 15,000 deaths, had a wind speed of more than 135 miles an hour with gusts reaching 160 miles an hour, said Laxman Singh Rathore, director general of the India Meteorological Department.

Cyclone Phailin, which Friday morning was around 300 miles from India’s coast, has reached a top speed of about 124 miles an hour, Mr. Rathore said.

India Meteorological Department

Chart showing wind speeds in Cyclone Phailin, Oct. 11, 2013.

“This is close to a super cyclone,” he said. “However, what matters is not just the wind speed, but the intensity of rainfall when it meets land.”

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